First, it must be understood that redo generation is at the
very heart of Oracle's ability to recover from virually any media
failure. For that reason, the ability to disable it only
exists for a small subset of commands. They are all related
to serial and parallel direct-path loads.

What operations allow NOLOGGING?
DML statements such as insert, update,
and delete will always generate redo.
However, the nologging option may be utilized for
the following SQL statements (from the Oracle 8i concepts manual,
chapter 22):

direct load (SQL*Loader)

direct load INSERT (using APPEND hint)

CREATE TABLE ... AS SELECT

CREATE INDEX

ALTER TABLE ... MOVE PARTITION

ALTER TABLE ... SPLIT PARTITION

ALTER INDEX ... SPLIT PARTITION

ALTER INDEX ... REBUILD

ALTER INDEX ... REBUILD PARTITION

INSERT, UPDATE, and DELETE on LOBs in NOCACHE NOLOGGING
mode stored out of line

For the statements listed above, undo and redo logging can be
almost entirely disabled. New extents are marked invalid,
and data dictionary changes are still logged. Note that a
table or index with the nologging attribute (which
can be seen in the LOGGING column of the DBA_TABLES or
DBA_INDEXES view) will default to nologging when
one of the above statements is executed.

What is the recoverability of objects created as NOLOGGING?
Since nologging disables writing redo log entries,
there is no way for Oracle to recover data related to nologging
operations. In the case of a media failure subsequent to a
nologging operation, Oracle will apply the redo log
transactions, but, when it reaches the transactions related to
the nologging operation, it will only be able to
apply the extent invalidation records, since that is all that was
recorded. Any subsequent attempt to access data in those
extents will result in an ORA-26040 "Data block was loaded
using the NOLOGGING option". The only resolution to
the error is to drop and recreate the object. Note that
this risk only exists until the next successful backup.
Backups taken after the completion of the nologging
operation will provide complete recovery.